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Adding downwards gives
which is 60 times the number of terms. How many terms are there? 53  7 = 46 hence you start with 7 and add 1, 46 times to get to 53 and hence you have 7 and then 46 more terms so there are 47 terms. Thus
or $S = \large \frac12 \normalsize 60 \times 47.$ You could also use the formula S = n[2a+(n1)d]/2 which is given in the resource you quote but I prefer to remember
All three of these techniques apply to a more general situation also. Both the sum of the first 100 whole numbers and your sequence start with a number and then add 1 to ge the second term, add 1 again to get the third term and so on. What is the number you add each time is not 1 but something else? For example start with 5 and add 3 to get the second term. Then add 3 again to ge the third term and so on.
What is this sum? Harley




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